I like the idea of the Legendaries sharing a type, especially if their second typing makes them super effective against each other. I just can't think of a combination that would fit...let's say they both are psychic. Yveltal does look like a flying type, Xerneas could be a grass type...but that would be unfair against Xerneas right?
On the other Hand, Kyogre was super effective against Groudon, so...